1. Field of the Invention
The present invention relates to a process for the preparation of aromatic and aliphatic aldehydes by catalytic gas phase hydrogenation of aromatic or aliphatic carboxylic acids or of some of their derivatives using a catalyst system based on titanium dioxide and/or vanadium oxide.
2. Description of the Related Art
It is known that aromatic and aliphatic carboxylic acids or carboxylic esters can be reduced by molecular hydrogen using various catalysts to give the corresponding aldehydes. For instance, U.S. Pat. No. 3,935,265 discloses that alkyl esters of aromatic carboxylic acids can be reduced by hydrogen at 400.degree.-600.degree. C. on Al.sub.2 O.sub.3 ; for example, methyl benzoate is converted to benzaldehyde with a selectivity of 37%, the conversion of the ester being 39%. Furthermore, it is known that zirconium dioxide alone (U.S. Pat. No. 4,328,373) or in combination with the oxides of other metals such as chromium, manganese, iron, zinc, cobalt, bismuth, lead, rhenium or main group III elements such as boron, aluminum, gallium, indium or thallium (EP 150,961) or together with oxides of elements of the lanthanide group (U.S. Pat. No. 4,328,373) is capable of reducing carboxylic acids or the esters thereof with hydrogen to the corresponding aldehydes. The abovementioned U.S. Pat. No. 4,328,373 furthermore discloses that, like zirconium dioxide, the oxides of yttrium, cerium, praseo-dymium, thorium and uranium are also effective, these like zirconium dioxide also being useable in combination with aluminum oxide.
U.S. Pat. No. 4,585,899 discloses that manganese dioxide in combination with aluminum oxide and silicon dioxide is also effective. EP 290,096 discloses that manganese dioxide can be deposited on various carriers such as aluminum oxide, zirconium dioxide, titanium dioxide, cerium(III) oxide, hafnium dioxide or niobium(V) oxide. No specific action of the carriers in combination with the manganese dioxide is mentioned in the above publication. According to EP 290,096, the type of preparation of the manganese oxide is crucial for the activity of the catalyst.
The effectiveness of a manganese dioxide/titanium dioxide combination is not described in EP 290,096. The present applicant's own experiments showed that a catalyst consisting of, for example, 23% by weight of manganese dioxide on titanium dioxide and prepared by coprecipitation of the components was not especially effective at 370.degree. C. for the abovementioned purpose. At higher temperatures, in particular above 400.degree. C., although the activity of the conversion of carboxylic acids or the derivatives thereof increased, the selectivity of conversion to the desired aldehydes declined to an industrially unacceptable level.